1. Field of the Invention
The present invention relates to an apparatus for polishing wafers and, particularly, to an apparatus for polishing semiconductor wafers relying upon a chemical-mechanical polishing (CMP) method.
2. Description of the Related Art
In recent years, ICs have been finely machined and IC patterns have been formed in a multiplicity of layers. Some degree of roughness is inevitably formed in the surfaces of the layers on which the patterns are formed. According to the prior art, the pattern of the next layer has been formed without any treatment. As the number of the layers increases and as the widths of the lines and holes decrease, however, it has become difficult to favorably form patterns and defects occur easily. Therefore, it has been attempted to form the pattern of the next layer after the surface of the layer on which the pattern is formed is flattened by polishing. The wafer-polishing apparatus (CMP apparatus) based on the CMP method is employed to polish the wafer in the step of forming the IC patterns.
There has been widely employed a wafer-polishing apparatus comprising a disk-like polishing table having a polishing pad stuck on the surface thereof, a plurality of wafer-holder heads that hold the surfaces on one side of the wafers to be polished and bring the other surfaces of the wafers into contact with the polishing pad, and a holder head drive mechanism for turning the wafer-holder heads relative to the polishing table, and wherein a slurry, which is a polishing agent, is supplied between the polishing pad and the wafers to polish the wafers.
As a wafer-holder mechanism in a wafer-holder head, there have heretofore been known one by which a wafer is adhered to a carrier via a wafer-adhering sheet (Japanese Unexamined Patent Publication (Kokai) No. 8-229808) and one by which a very finely porous insert having elasticity is adhered to the carrier and the wafer is held by being adhered thereto (Japanese Unexamined Patent Publication (Kokai) No. 6-79618).
According to the above-mentioned conventional wafer-holder mechanisms, however, a so-called packing sheet must be stuck to the surface of the carrier, and bubbles evolve at the time of sticking the packing sheet requiring skill for sticking, the degree of flatness on the surface on which the packing sheet is stuck affects the wafer-machining surface, and the wafer-holder head must be removed for sticking the packing sheet.
The applicant therefore has proposed an apparatus for polishing wafers in which an air-blowing member is provided under the lower surface of the carrier that is loosely supported in a wafer-holder head body so as to move up and down, in order to blow the air toward the back surface of the wafer thereby to form a pressurized fluid layer between the carrier and the wafer, and the wafer is held being pushed onto the polishing table via the pressurized fluid layer, as taught in Japanese Patent Application No. 10-92030 filed by the applicant.
Even in the wafer-holder mechanism in the above-mentioned apparatus for polishing wafers, however, the back surface of the wafer is likely to come into direct contact with the hard surface of the carrier and is scratched while the wafer is held by adsorption and is polished.